Installed new front and rear O2 sensors and they check out fine using a good OBII scanner. Before I installed the 02 sensors the codes indicated the sensors were bad so I replaced them. The new ones now indicate they work fine. The one code I do get now is P0420. The check engine light only displays when it is pretty cold outside and even after I have driven it a long time and it is warmed up completely it stays on. Today it is warmer outside and it is not displaying at all and it is running a bit smoother. I do not visibly see an exhaust leak or feel one. According to this code it could be the coolant temperature sensor. Has anyone had this issue? Could I still have a small exhaust leak? I should would like to figure out for sure if my catalyst is failing or not since a good one cost some $$.

All fluids are fine too as well as the gas cap. Spark plugs are good. No noises.

I should also mention, I installed these 02 sensors a few months ago while the weather was warm and have not had any engine lights or any other issues until it just began to get cold outside so my thinking is that the sensors are fine. Also, I am aware that if you drop these sensors or bang them at all that this may damage them. I was carefull not to drop or bang them or anything when installing them.